A Mechanism of Coupled Circulation Leading to the Secondary Eyewall Formation Associated with Hurricane Katrina (2005) Using Real and Idealized Numerical Simulations

Based on a thorough review of previous studies of secondary eyewall formation (SEF) in tropical cyclones, we hypothesize that a coupled circulation composed by (1) an increase of divergence outside the primary eyewall of the hurricane at the mid- to upper-levels, (2) a downward motion outside the primary eyewall, which helped the development of a pronounced “moat” region, and (3) an outer-ring of convergence forming above the boundary layer is the key mechanism leading to SEF.

This hypothesis is first tested by performing real-case analysis and simulations of the last SEF of Hurricane Katrina (2005) using the Advance Research WRF (ARW) model. We found that the SEF began above the boundary layer in association to persistent downdrafts outside the primary eyewall, which acted to force the development of a secondary convergence zone at the radius where the secondary eyewall eventually developed. In addition, the strong radial inflow within the boundary layer inhibited the penetration of the downdrafts thus explaining the initiation of SEF above it. Once the sinking air mass encountered this relatively rigid lid it had two main horizontal radial components: inward and outward. This last component was the one responsible for the enhancement of convergence and an eventual acceleration of the tangential winds in the SEF radius. A pre-existing area of persistent rainbands and/or a strong feederband wrapping around the inner core was also required for the SEF. The beginning of a persistent downdraft may be traced mostly to the mid to upper levels, though an initial influence on SEF from the lower-levels is not discarded. Once the updraft associated with the strengthening primary eyewall becomes very significant, the balancing response to it is to intensify the downdraft outside the eyewall which will eventually force the mechanism mentioned above. Consistent with this is an observed drying of the moat region in between the primary eyewall and SEF regions. In addition, the hypothesis is tested by idealized numerical simulations which provide simpler, more controllable environments.